Polycyclic aromatic hydrocarbons (PAHs), also known as poly-aromatic hydrocarbons or polynuclear aromatic hydrocarbons, are potent atmospheric pollutants that consist of fused aromatic rings and do not contain heteroatoms or carry substituents. Naphthalene is the simplest example of a PAH. PAHs occur in oil, coal, and tar deposits, and are generally produced as byproducts of fuel burning (whether fossil fuel or biomass) or incomplete combustion or pyrolysis of organic matter. Due to the great number of applications of combustion in our daily lives (e.g., heating, cooking, fossil fuel burning, cigarette smoking and etc., PAHs are formed in great abundance. In addition, since each of these combustion processes occurs at various temperatures and under various environmental conditions, different PAHs are formed. Emitted PAHs generally absorb on the surface of soot particles, thus allowing the dispersion of these compounds throughout the environment. As a pollutant, PAHs are of concern because a number of them have been identified as carcinogenic, mutagenic, and teratogenic.
Currently, there are twenty-four PAHs that have been identified as being hazardous to human health. See Table 1. The EU Scientific Committee for Food (SCF), the European Union (EU), and the US Environmental Protection Agency (EPA) recommend a frequent monitoring of these twenty-four PAHs. This list is expected to grow longer as more PAHs with potentially toxic properties are identified. Therefore, there is a need for an efficient monitoring of PAHs in the environment and in almost anything (e.g., food, beverages, packaging materials, medical devices, etc.) that may increase the risk of human exposure to these compounds.
TABLE 1Names and structures of frequently monitored PAHs. (source: PolycyclicAromatic Hydrocarbons (PAHs) Factsheet, 3rd edition, European Comission, Joint Research Centre; Institute for Reference Materials and Measurements;European Union 2010)ListCommon NameStructureEPA, SCF, EUBenzo[a] pyrene EPAAcenaphthene EPAAcenaphthylene EPAAnthracene EPA, SCF, EUBenzo[a] anthracene EPA, SCF, EUBenzo[b] fluoranthene SCF, EUBenzo[j] fluoranthene EPA, SCF, EUBenzo[k] fluoranthene EUBenzo[c]fluorene EPA, SCF, EUBenzo[ghi] perylene EPA, SCF, EUChrysene SCF, EUCyclopenta [cd]pyrene EPA, SCF, EUDibenzo[a,h] anthracene EU + SCFDibenzo [a,e]pyrene EU + SCFDibenzo [a,h]pyrene EU + SCFDibenzo [a,l]pyrene EU + SCFDibenzo [a,l]pyrene EPAFluoranthene EPAFluorene EPA, SCF, EUIndeno[1,2,3- cd]pyrene EU + SCF5-Methyl chrysene EPANaphthalene EPAPhenanthrene EPAPyrene
Typical methods for analysis of PAHs include HPLC (high performance or pressure liquid chromatography) and GC (gas chromatography). However, there are shortcomings associated with each of these methods. For example, the GC methods only detect volatile compounds and non-volatile compounds require derivatization prior to a GC analysis, which is burdensome, expensive and time-consuming. In liquid chromatography methods, although no sample derivatization is required, the typical run time of a sample on an HPLC instrument is about 25 minutes; which has recently been reduced to about 10 minutes by using a UHPLC (ultrahigh performance or pressure chromatography) instrument. There are other disadvantages to using HPLC or UHPLC, one of which is their using of toxic organic solvents as mobile phase and generating toxic waste, which is expensive to purchase and dispose of.
The use of non-toxic Supercritical CO2 (SC—CO2) as an alternative to organic solvents as the mobile phase has resulted in the advent of supercritical fluid chromatography (SFC) which embraces many of the features of liquid and gas chromatography. Theoretically, SC—CO2 provides a low viscosity mobile phase that achieves higher diffusion rates and enhanced mass transfer over the solvents used in HPLC. However, the current SFC instruments (which are mainly retooled HPLCs) and methods have many limitations including, for example, long sample run time, inaccurate or imprecise control over the mobile phase density and composition, inability to reliably deliver modifiers at low amounts (<5% of liquid CO2), susceptibility to system pressure fluctuations and sample backflow, baseline noise, sample carryover, and lack of robustness, which prevent users from rapidly obtaining reproducible results.
Therefore, there still remains a need for a more improved chromatography system and method that can overcome the above limitations and allow for a rapid and robust analysis of PAHs.